With the spread of the Internet, a volume of data for communication has increased. In order to cope therewith, it has become necessary to increase a capacity of a transmission path. As one of techniques for realizing large capacity, there is a multi-value modulation signal (polarization division multiplexed-quadrature amplitude modulation: PDM-QAM). An optical signal on which the PDM-QAM modulation has been performed in a transmitter is demodulated in an optical receiver of a digital coherent method (for example, refer to Patent Document 1).
In addition, Patent Document 2 discloses that a signal obtained by adding an output from a slicer to an input to the slicer, and a signal obtained by making the output from the slicer pass through a division device, are used as an input to a feedback filter.
Further, Patent Document 3 discloses that a filter coefficient of a digital filter is controlled in a feedback manner.